Building an AR pistol is very much the same as building an AR Rifle. The only big difference is that the buffer tube can not be able to accept a stock.
Barrel length is up to you. There is no barrel length restriction for a pistol. The barrel can be as short as you can make it (and still function of course) 7" is the shortest that cycle with a pistol length gas tube. A barrel can be as long as you want to make it. The definition of a pistol is not in the barrel length.
All the other AR parts are pretty interchangeable with an AR pistol. The bolt, the upper receiver, the lower receiver are all the same. The gas tube is dependent on the barrel length you decide to use, of course.
The only "caveat" is that a pistol lower can have NEVER been built into a full rifle. You can use a stripped lower, or even a complete lower that HAS a stock on it, but has NEVER been built into a full rifle. You must remove the stock before putting an upper on it. (Once you put an upper on a lower, it is a complete RIFLE, and can NOT be a pistol any longer.)
This is the one I built: